In radiation-emitting semiconductor components, the internal conversion efficiency of electrical energy into radiant energy is usually much higher than the overall efficiency. This is due essentially to the low outcoupling efficiency from the semiconductor component of the radiation generated in the active zone. There are a number of reasons for this. It is frequently desired for current to be injected into the semiconductor layer sequence over a large area, which can be achieved for example by means of large-area metal contact structures. However, such contact structures usually are not transparent to the generated radiation and cause high absorption thereof.
There are also ways to inject current over a large area using small-area contact structures that do not cover the semiconductor body completely. For this purpose, the radiation-emitting semiconductor component can, for example, comprise so-called current spreading layers, which provide uniform current injection into the active zone. This can be achieved, on the one hand, by means of layers of doped semiconductor material disposed in the semiconductor layer sequence. Such layers must be relatively thick to ensure uniform current injection into the active zone. The thicker the semiconductor layer, however, the greater the amount of time needed to make the layer sequence. Furthermore, as the layer thickness increases, so does the absorption of free charge carriers and/or of the generated radiation in these layers, resulting in low overall efficiency.
In addition, known from JP 2000-353820 is a component having a current spreading layer that is transparent to the generated radiation. This layer contains ZnO, which belongs to the TCO (Transparent Conducting Oxides) class of materials. In addition to ZnO, ITO (Indium Tin Oxide) is another material from this class that is commonly used for current spreading.
The outcoupling efficiency is further limited by the total reflection of radiation generated in the active zone from interfaces, owing to the different refractive indexes of the semiconductor material and the surrounding material. Total reflection can be disturbed by suitable structuring of the interfaces. This results in a higher outcoupling efficiency.
Another cause of low outcoupling efficiency is absorption of the radiation in a substrate or a carrier on which the semiconductor layer sequence is grown or the radiation-emitting semiconductor component is mounted.